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Below is a list of links to the main pages about our yacht, Catlypso and our Our Yachting Adventures:
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    Michael and Kelly's 4WD Trips
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    Current Kareela Weather
    A summary of the current weather conditions at our house at Kareela, Sydney, is below. Click here for more Detailed Diving Weather and Conditions. Weather from Michael McFadyen's Tempe Weather Station


    Conditions at
    23:49 on 23/1/17

     
    Temperature 25.6°C
    Humidity 65.0%
    Barometer 1003.4hPa
    Rate -0.3hPa/hr
    Wind Speed: 0 km/hr
    Wind Direction S
    Rainfall for Today 0.0mm
    Rainfall last hour 0.0 mm
    Rainfall last 24 hours 0.0 mm
    Rainfall at Start of Month 814.6 mm
    Rainfall this Year 827.0 mm
    Today's Extremes
    High Temperature 31.2°C at 15:48
    Low Temperature 20.9°C at 6:27
    Peak Wind Gust 0km/hr at 0:00
    Weather from Michael McFadyen's Kirrawee Weather Station
    Yesterday's Extremes
    High Temperature 27.5°C at 17:11
    Low Temperature 19.8°C at 6:29
    Rainfall at Start of Yesterday 827.0 mm
    Rainfall at End of Yesterday 827.0 mm
    Weather from Michael McFadyen's Tempe Weather Station
    Astronomical Data
    Sunrise 5:06
    Sunset 19:05
    Moonrise 1:09
    Moonset 15:04

    Home Brewing
    Click here for an article about Home Brewing.
    Sydney Dive Site Hints
    "Cape Banks has lots of swim-throughs"
    Wave Period and Diving and Boating
    For diving, the sea condition is of course one of the most important variables to be taken into account when considering where and when to dive. For example, totally flat seas are always safe unless there is a raging current, although even this may not be a limiting factor if you are planning on doing a drift dive.

    Accordingly, one of the things that we as divers need to consider is the weather and more importantly, the sea conditions (of course I am writing about Australia where 99.99% of dives are done in the ocean or large bays or harbours.

    The sea conditions can affect divers whether you are diving from the shore or on a boat. The following is mainly related to boat diving and does not consider the weather conditions (eg wind speed or direction). While some sea heights may not be large, the type of waves can be totally unsuitable to taking a boat out in the open seas or even on the bay or harbour.

    Waves can be big or large, rough or smooth. Off Sydney (where I live), the seas can vary from 0 metres to well over 12 metres at times. Of course the large waves are dangerous and would be unsafe for boating or diving. At the same time, small waves of only one metre can be unsafe for boats if the waves are very choppy. Likewise, larger waves of 4 metres can sometimes be safe for boat if the wave is smooth and a long rolling one. However, this wave is probably not suitable for diving, even at depth (which I will show later).

    Some stories first. Back on Australia Day 1996 (26 January), I did a dive on the wreck of the Tasman Hauler south of Eden. We had a reasonable dive. The conditions on top were a very large swell but at the bottom (30 metres) it was comfortable. After lunch we came back for a second dive. The conditions on the top appeared similar to the first dive. However, on the bottom the scene was totally different. There was a huge surge, so much so that the visibility had dropped from 5 metres or so to less than a metre and I actually was physically thrown off the wreck and could not find it again.

    The difference between the dives was that the period of the wave (the time between the wave peaks) had increased dramatically. My guess is that it had changed from about 10 seconds to about 15 seconds in the five hours between the dives.

    Another example is the damage caused to wrecks like the SS Tuggerah which is at over 45 metres. I have dived the wreck a week after huge seas and found pieces of steel twisted by the wave action. Even diving this deep we have at times been pushed 5 or 10 metres backwards and forwards by waves that appeared to be not too big.

    How to work this out?

    Well, have a look at the next table.

    Wave Period
    Seconds
    Wave Length
    Metres
    Wave Speed
    Km/h
    Depth of surge
    Metres
    8100 45 16
    10156 56 25
    12225 67 36
    14306 79 49
    15351 84 56
    16399 90 64
    17451 96 72
    18505 101 81

    Boating
    If you have access to a wave recording buoy via the internet, then have a look at the graph for the area you are planning to dive and see what the period is. If the seas are over about 1.5 metres and the period under 6 seconds, then I would not take my boat out as it would be far too choppy. Smaller waves with shorter periods would also be unsuitable. See later about diving.

    Diving
    For diving, the effect is not so obvious . Again, look at a wave recording buoy. If you do not have access to one, then an easy way to work it out if you are out on the water is to count the time between waves reaching you once you are stopped or anchored. Then, use the table above. If you do not have the table, then square the time and divide by four. This gives a rough result as above.

    Therefore, if you are planning to dive the SS Myola or SS Birchgrove Park which are both at about 50 metres, then you need the period of the waves to be under about 15 seconds. At 15 seconds you would feel some surge. Under 13 seconds you should feel none.

    If you were planning to dive Middle Ground (just over 30 metres), then the 15 second period would be very uncomfortable and 12 seconds would mean you could feel the surge a bit. Ten seconds would have no surge. Also note that there while most wave periods will be constant, there will be some that are longer than the average period which will reach deeper. On 11 May 2011 I dived Middle Ground and there was a period of 8 secods average but some were 12 seconds. We could occassionally feel the surge.

    Of course, this is all theory, but for us seems to work.

    Copyright © Michael McFadyen 1990 to 2017
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    This web site has been wholly thought up, designed, constructed and funded by Michael McFadyen
    without any help from the Australian Dive Industry since 1996!